1. Field of the Invention
This invention relates to a device for preventing substantial static charge buildup on a substrate surface and, in particular, to a precipitative static drain strip system for dissipating static charge on the outboard surface of a laminated aircraft window or add-on window (appendage).
2A. Technical Considerations
Windows used in aircraft typically include several glass and/or plastic layers or plies which are fused or laminated together to form a composite transparent window. As used herein, the term composite ply, transparency, windshield, or window refers to a substrate containing one or more glass and/or plastic layers. In order to remove fog, mist or ice which deposits on the outside or outboard surface of the window during flight, the aircraft windows may include an internally-located electrical device for heating the window. In addition to fog or ice, static electric charge also collects on the outboard surface of an aircraft window during flight. Air flowing over the outboard surface, as well as frictional contact with atmospheric particles such as snow or dust particles striking the window, will deposit an electrical charge on the window surface. The amount of charge that may be accumulated depends on the dielectric strength of the outboard ply of the window as well as the size, i.e. surface area, of the window. For large windows, potentials of 50,000-100,000 volts may be reached. If allowed to build up without some type of auxiliary grounding arrangement, the window may fail due to the static charge reaching an electric potential sufficient to discharge through the window to the interior heating device. Scratches and normal service deterioration of the window which adversely affect the strength of the window and/or reduce the window thickness could conceivably reduce the dielectric strength to one-half or one-third of its original strength, thus reducing the electric potential required to discharge through the window.
More particularly, a direct static charge accumulation results in flashover that punctures small holes in the outboard ply and discharges into the heating circuit of the window, damaging the ply, the heating system filament wires, (if applicable) and the interlayer in which the heating circuit is situated. It is, therefore, necessary to provide some facility for dissipating static charge before it reaches an electric potential value sufficient to discharge through the window.
A method for preventing static charge buildup on the outboard surface of an aircraft window is to coat the outboard window surface with an electroconductive anti-static coating or film. A problem with coating the window surface with these anti-static coatings or films is that the Federal Aviation Regulation Part 23 which applies to general aviation aircraft currently requires a minimum of 70% light transmittance through the total viewing area of the windshield, i.e. 70% of the visible light range must be able to enter the aircraft through the windshield. Windshields having presently available coatings and films will pass less than 70% of the visible light so that this Federal Aviation Agency regulation is not met.
It is therefore desirable to have techniques for effectively dissipating static charge from an aircraft windshield while meeting FAA light transmittance regulations.
2B. Patents of Interest
U.S. Pat. No. 4,078,107 to Bitterice et al. teaches a lightweight aircraft window having a heating circuit embedded the full depth of the outboard plastic ply and an anti-static circuit embedded just beneath the outboard surface of the window. The anti-static circuit provides an electrically conductive path of thin static-charge dissipating wires parallel to the outboard surface of the window covered with a plastic overlayer for dissipation of static charge before the charge reaches a level sufficient to cause electrical discharge from the outboard surface of the window to the interior heating circuit. It should be noted that because the charge collected on the window surface must travel through the plastic overlayer to reach the dissipating wires, the charge can still create small holes or punctures therein.
U.S. Pat. Nos. 4,323,946 to Truax and 4,590,535 to Mang each teach static charge dissipating circuits wherein electroconductive members extend at least partially through the thickness of the outer windshield ply to the outer surface. The inboard end of these electroconductive members are interconnected by way of either longitudinally extending wire members embedded in the outboard ply or an electroconductive surface at the interface between the outboard ply and the next inboard ply.
U.S. Pat. No. 4,382,881 to Levy discloses an anti-static coating exhibiting a desired value of surface resistivity, but which is substantially free from metal particles. This type of coating is used on the surface of nonmetallic elements of an aircraft such as radomes protecting an antenna, which if made too conductive, may become opaque to electromagnetic radiation by modifying their radio-electrical transparency.
U.S. Pat. No. 4,571,361 to Kawaguchi et al. teaches an anti-static film including a coating layer having electrically conductive fine particles dispersed in a compound containing an unsaturated bond capable of undergoing polymerization via radiation or a binder containing a polymerizable compound. The resulting dispersion is coated on the plastic film, polymerized, and cured via radiation.
U.S. Pat. No. 4,607,313 to Shaw et al. teaches a static discharger for discharging charge accumulation from the surface of the wing of an aircraft. A generally flat, non-conductive member is configured to conform to the shape of a portion of the wing surface. A flat base member is configured so as to be capable of being placed in conforming relationship with a least a portion of the surface of the non-conductive member which electrically insulates the base member from the surface of the aircraft. A plurality of strands formed of discharging material are embedded in the base member and are electrically connected to the wing surface of the aircraft so that charge accumulation passes from the wing surface of the aircraft to the plurality of strands to be discharged at the trailing end of the strands.